1. Field of the Invention
The present invention relates to a video circuit, or more particularly to a video circuit for compensating for the deterioration of picture details of a low luminance signal in the case where a high luminance signal and a low luminance signal are mixed.
2. Description of Related Art
FIG. 1 is a block diagram showing a conventional video circuit.
A composite video signal such as a television signal is separated into a chrominance signal and a luminance signal at a Y/C separator circuit 101. The chrominance signal is supplied to a video signal processing circuit 102 and the luminance signal to a DC transmission rate correction circuit 103. The DC transmission rate correction circuit 103 compensates for the deterioration of the details of the video signal of a low-luminance signal, and is operated so that a pulse signal having an amplitude corresponding to that of an average luminance level is extracted from the luminance signal, is added to the luminance signal and is supplied to the video signal processing circuit 102 as a transmission rate-corrected luminance signal. The video signal processing circuit 102 generates three primary color signals from the transmission rate-corrected luminance signal and the chrominance signal and applies it to a CRT 104.
Generally, in the case where a high-brightness signal is generated at more than a predetermined ratio with respect to a low-brightness signal by the camera imaging process, the total quantity of light is corrected by the "aperture". As a result, when such a signal is received, a phenomenon called "black deterioration" develops in which the video portion of a low-brightness signal is deteriorated in black. This is because the pedestal level of the video signal is moved toward dark side. Historically several suggestions have been made to cope with this problem.
The DC transmission rate correction circuit 103 is well known and shown in the block diagram, which constitutes a part of a bipolar IC designed for processing the luminance system signal for the video signal. The luminance signal is supplied to an APL (Average Picture Level) detector 131 and a transmission rate corrector 133. The APL detector 131 removes the horizontal sync signal portion of the inputted luminance signal, which is smoothed to an average luminance signal level by a resistor and a capacitor (not shown) connected to the transmission rate correction terminal 132. The resulting signal is supplied to a transmission rate corrector 133. The transmission rate corrector 133 generates a transmission rate-correction pulse having an amplitude corresponding to that of the average brightness level signal. The transmission rate-corrected pulse thus generated is added to the back porch of the pedestal level of the luminance signal supplied to the transmission rate corrector 133 thereby to produce a transmission rate-corrected luminance signal, which is supplied through an output buffer 134 to the video signal processing circuit 102.
FIG. 2 is a diagram showing a waveform of the transmission rate-corrected luminance signal. The level of the bottom section of the transmission rate-corrected pulse is processed to function as a pedestal level in the video signal processing circuit 102. As a result, the larger the amplitude of the transmission rate-corrected pulse, the brighter the luminosity of the screen is corrected for reproducing the image details of a low-brightness signal.
As described above, according to the conventional video circuit, the transmission rate-corrected pulse is added to the back porch, which is so close to the sync signal that the separation thereof is difficult, thereby sometimes causing an unstable synchronization. Also, the conventional DC transmission rate correction circuit 103 is operated to affect correction in such a direction as to attain an even higher screen luminosity, the higher the brightness of a signal.
When the luminosity of the CRT 104 exceeds a predetermined level, an excessive current flows in the CRT 104. This causes an excessive load on the high-voltage generator and damages the horizontal output transistor. In order to prevent this malfunction, the video signal processing circuit 102 includes an automatic brightness limiter which detects the change in the current of the CRT 104 and, when it is excessive, the current of the CRT 104 is limited to below a specified value by a negative feedback. When the current in the CRT 104 is reduced excessively, the automatic brightness limiter regulates the CRT 104 in such a manner as to increase the current thereof. There occurs a time lag due to the increase or decrease in the current of the CRT 104 as a result of the negative feedback. When the brightness change is abrupt causing abrupt correction, the automatic brightness limiter is likely to over compensate, followed by excessive relaxation, thereby causing current oscillation on the CRT 104.
In addition, the transmission rate-corrected luminance signal is correspondingly determined by the circuit constants of the DC transmission rate correction circuit 103. Therefore, the correction characteristics of color saturation cannot be set. As a consequence, the correction of the brightness level is liable to cause insufficient color saturation correction.